Hey, I think your idea is relatively good but I do have a few problems with it. Firstly, SCP xxxx-2 is originally mentioned in the unamended containment procedures, but later it is stated to be discovered in Addendum 01 and classified as SCP xxxx-2 in Addendum 02. Therefore as these tests were done after the original entry, there should be an Addendum to the containment procedures as well noting the special containment of xxxx-2, and ditto all that for xxxx-3. lastly I think that a further Addendum that explains if xxxx-2 is radioactive, viral or any other guess that the staff that reordered (ie, nanites, symbiotie or whatever) might be a nice addition, and maybe some tests on other life forms. Hope any of this is helpful.

Firstly, your sandbox should be named after yourself, not after your object.

Item #:XXXX

You should always refer to it as "SCP-XXXX", never just by its number.

Object Class: Elucid

It's spelled "Euclid".

Special Containment Procedures: SCP-XXXX-01 is to be kept in a locked strongbox of 30x30x30 cm, inside a strong room with an independent, dry atmosphere with 0% humidity.

Unless there's a specific reason the box needs to be a cube 30 centimeters on a side, then there's no need to specify.

"Strong room" is not a thing.

If the room needs to have 0% humidity, then calling it "dry" is redundant.

If any wish to utilize/experiment with the object, a written request must be submitted to Dr. Anderson.

Containment procedures should never be dependent on a specific individual, since it implies that nobody can ever test the object again if that person dies, retires, quits, goes on vacation, or just plain calls in sick one day.

All researchers are to wear heavy-duty HAZMAT gear while handling the object, and all research is to take place under supervision of Class-3 Security Personnel.

Don't call it "the object". Call it "SCP-XXXX".

"Class-3 Security Personnel" are not a thing.

SCP-XXXX-2 is to be kept in a cold storage vault at Site 19.SCP-XXXX-02 is to be kept in a high security cold storage vault, at -20o Celsius.

The second sentence restates the first, and you're missing a space between the sentences.

A black substance has been poured in small amounts om the top of its head and wings

"On" the top.

Object was discovered in 1976 at [DATA EXPUNGED].

You should be telling us what the object does before telling us where it was found.

If you must refer to it as an object, call it "the object", not "object". Trimming articles from your sentences doesn't make them sound any more science-y.

Object was found to be in the "hand" of a large, mutated biomass. upon further investigation, 37 more similar biomasses were found within 100 meters of the main building.

Missing capitalization.

"The main building" of what?

When biological matter comes in contact with XXXX-2, rampant mutation takes place at the site of contact.

Never drop the "SCP-" from the term.

What's "XXXX-2"? You haven't defined any such thing yet.

That's funny, seeing as you didn't mention anything in the containment procedures about it being held in a sterile facility, and therefore there are any number of bacteria, wild yeast, or other microscopic nasties just floating around in the air waiting to set it off.

Responding containment teams have disposed of all evidence in the area. Object has been logged.

Why is this written in present tense?

Testing of SCP-XXXX has yielded disturbing results.

Subjective adjectives such as "disturbing" have no place in scientific writing.

The black substance (originally thought to be water-soluble ink) has shown to be genetic material of an unknown source.

"Genetic material"?

test results have concluded that the black substance was the cause of the aforementioned rampant and uncontrollable mutation. The material is very easy to reproduce, as any contact to water results in rapid reproduction. and has been stored under SCP-XXXX-02.

Multiple missed capital letters.

Information hidden in an addendum that should belong in the description.

Again, the containment procedures never say anything about keeping SCP-XXXX-02 in a water-free environment.

Approximately seven (7) hours after direct exposure

Why are you enumerating your numbers now when you haven't done so at all in the entire rest of the article?

the subjects muscle and bone mass increases 200%.

This is frankly ludicrous, since the carrying capacity of bone doesn't scale in a linear fashion with its mass, but is rather related to surface area. As an organism becomes more massive, it requires exponentially bigger and thicker bones to support its weight - there's a reason that an elephant's femur is thicker around than a human being's entire leg.The creature you describe would barely be able to walk without shattering its femurs.

SCP-XXXX-3 shows a preference to human flesh above all, and have been observed to exert great strength to reach it.

At no point have you defined an SCP-XXXX-3.

Final thought: Black goo that makes scary monsters that kill everyone FOR NO RAISIN, and an angel statue that seems to have nothing to do with anything, since you never describe what it does or how it relates to the black goo. Confusing and muddled with bad tone, bad science, and bad grammar. Would downvote.